With the increasing development of communications devices and the expanding variety of those devices, there has been an increased demand to locate associated antennas in areas that improve the performance of the communication devices. In motor vehicles, concerns with aesthetics and not obstructing the view of the vehicle occupants further limit the range of desirable locations for antennas. Typical communication devices that are now installed in many vehicles include AM/FM radios, cellular phones, and vehicle remote keyless entry systems. Also, vehicle remote start-up systems are projected to enter the market in the near future.
Generally, traditional remote keyless entry systems include an internal antenna to receive radio waves that direct the system to lock or unlock the vehicle. The keyless entry module is usually located under the dashboard within the passenger compartment or other locations within the vehicle. Typically, these systems have provided satisfactory performance, however reception of the vehicle lock/unlock signal is limited due to attenuation of the signal by the intervening vehicle structure. Additionally, intermittent reception of the lock/unlock signal occurs depending on the direction from which the signal is transmitted due to differences in the amount of vehicle structure that intervene between the signal source and the internal antenna. For example, the keyless entry system might be unresponsive to a signal transmitted from near the back quarterpanel, but be responsive to a signal transmitted from near the driver's side door. Intermittent or inconsistent operation is undesirable because of the possibility of customer dissatisfaction.
An alternative to mounting an antenna in the passenger compartment is to mount the antenna in the windshield. Traditionally, mast or whip antennas have been used to receive and transmit radio waves from a motor vehicle. Recently, thin film antennas that are affixed to a vehicle window have been developed (see U.S. Pat. Nos. 5,083,135, 5,528,314, and 5,739,794). The thin film antennas have eliminated the need for mast or whip antennas for VHF and UHF reception.
U.S. Pat. No. 5,083,135 issued Jan. 21, 1992, describes a transparent film antenna for a vehicle window in the shape of a "T" with a horizontally elongate principal element spaced from and parallel to the upper horizontal edge of the window and a vertically elongate impedance matching element extending down the window from the center of the principal element. A disadvantage associated with film antennas wherein the elements extend into the main viewing area of the windshield is the impact on vehicle appearance resulting when sunlight reflects off of the surface of the windshield. The sunlight tends to have a shimmering effect on the area of the windshield where the film antenna is embedded.
U.S. Pat. No. 5,528,314 issued Jun. 18, 1996 and U.S. Pat. No. 5,739,794 issued Apr. 14, 1998, provide improved film antennas (referred to as solar-ray antennas) that do not degrade the appearance of the vehicle. Both solar-ray antennas include a horizontally elongate principal element spaced from and parallel to the upper horizontal edge of the window. Each antenna includes an impedance matching element coupled to the principal element and extending outward so that it is spaced from the edge of the window aperture so as to form a slot transmission line. The impedance matching elements therefore enclose or occupy almost the entire windshield with the exception of a strip across the top in the area that is normally tinted.
A vehicle that employs a solar-ray antenna for AM/FM reception precludes using a conventional film antenna for a vehicle keyless entry system because of physical interference. The solar-ray antenna encloses virtually the entire windshield except for the tinted area at the top. A vehicle keyless entry system that employed a conventional film antenna located in the tinted region of the windshield would require a vertical impedance matching element that would extend into the impedance matching element of the solar-ray antenna. If the solar-ray impedance matching element is shifted to a lower position on the windshield so as to avoid physical interference with the keyless entry system antenna, the appearance of the vehicle will be degraded due to the shimmering effect of the windshield area with the film antenna contrasting with the remainder of the windshield. Accordingly, what is desired is a film antenna that can be located entirely within the tinted area of a windshield.